It is well-known in the art to provide magnetic particle brakes and clutches as shown and described in U.S. Pat. Nos. 4,575,103 and 4,681,197. In general, these devices are constructed with a gap between two armatures or poles and a shaft-mounted rotary member extending into the gap in spaced relation to the two armatures. The gap between the opposite sides of the rotary member and the adjacent armatures is filled with magnetic particles which function as the clutching medium. Under application of a magnetic field between the armatures, the normally fluid particles become a gel-like mass, thereby applying frictional forces to the rotary member and, consequently, to the shaft.
Hysteresis brakes and clutches are generally similar in construction to such magnetic particle devices, except that the magnetic particles are eliminated and magnetic forces rather than frictional forces provide the clutching medium. The rotary member of the magnetic particle devices is replaced by a shaft-mounted, magnetizable drag cup that rotates in a gap between the poles of a primary magnet which can be either a permanent magnet or an electromagnet. These devices have a number of advantages over the prior art magnetic particle brakes and clutches, in particular eliminating the problem of confining the magnetic particles inside the gap. These advantages include long life, environmental stability, precise repeatability and constancy of performance.
Since torque is produced without physical contact of parts, hysteresis devices are not subject to wear (except the normal wear of anti-friction bearings). This feature makes them distinctly superior to mechanical-friction brakes and clutches in life expectancy, servicing requirements and constancy of performance. Hysteresis brakes and clutches are also the most repeatable braking and clutching devices known. They will repeat their performance precisely, an indefinite number of times, whenever operating factors are repeated.
Hysteresis clutches and brakes are also stable in practically any environment. They are not damaged by reasonable temperature cycling, and can operate as hot as oil and bearing lubricants will tolerate. Hysteresis units have high heat-dissipation capability. When operating by fixed current they show no significant torque variation even in extreme ambience. They have operated at -100.degree. F., and the rotor member has proved stable at higher than +1000.degree. F. They also have the widest speed range of all electric torque-control devices, from zero to a high speed determined by kinetic power dissipation and the physical size of the unit. By way of example, a 1 inch diameter brake operates to speeds of over 30,000 RPM. Because they do not depend on mechanical friction, hysteresis units are absolutely and constantly smooth at any slip ratio.
Hysteresis units have high torque-to-signal linearity below saturation (except near zero). Their power consumption is extremely low. Since their working members have no physical contact and thus can accept moderate expansion without effect on operation, they can be readily adapted for use in high-vacuum applications.
Unfortunately, however, hysteresis brakes and clutches also suffer from a problem not experienced by the magnetic particle devices. Under certain conditions, hysteresis brakes and clutches experience a salient-pole phenomenon called "cogging", an undesirable, pulsating output torque that prevents smooth and efficient operation of these systems.
By way of example, consider the operation of a typical hysteresis brake. If the brake's input shaft is rotating and power is simultaneously applied to the coil, the brake will act on the shaft to slow it down in a smooth and regular fashion, without any cogging occurring. Power can also be safely reduced to the coil while the shaft is rotating without any cogging occurring. It has also been found that if the shaft stops (or substantially stops) with no change in power to the coil, no cogging will thereafter occur. However, if the power input to the coil should be reduced while the brake's input shaft is stationary, or substantially stationary, the brake will thereafter exhibit a cogging effect until it is "de-cogged". The phenomenon is such that the more power is reduced to the coil while the shaft is stopped, the more pronounced the cogging effect will be. Unfortunately, some manufacturing processes require stopping the shaft and reducing power to the coil, in exactly the fashion that leads to cogging. Thus, in some manufacturing processes, cogging can be a serious problem.
The "cogging" effect appears to be caused by the interaction of the poles of the primary magnet and the magnetized cup when the unit is de-energized. The residual impression of alternating north-south polarities around the circumference of the drag cup try to align with the closest corresponding pole pair of the primary magnet. The result is that the cup, and consequently the shaft, resists turning in either direction until external torque rotates the cup a sufficient angular amount to cause the poles of the cup to try to align with the next sequential pole pair of the primary magnet. This is experienced as a pulsating, cogging torque that can be as high as about one-third of the full rated torque. The existence of the "cogging" effect thus limits the utility and desirability of hysteresis brakes and clutches.
One way to eliminate "cogging" (at least in the case where the primary magnet is an electromagnet) is to first increase the input electrical power to the electromagnet to the highest value previously used and thereby reestablish the highest output torque before cogging occurred. Second, it is necessary to decrease the input electrical power to the electromagnet to zero while simultaneously turning the cup and shaft about one complete turn. This procedure has the effect of demagnetizing the cup, thereby preventing the cogging effect. However, this procedure is frequently inconvenient or impossible to perform, especially if the hysteresis device is mounted remotely from the operator, such as being located inside another piece of machinery. Furthermore, torque output during the process may be undesirable.